In the production of integrated circuits such as VLSI circuits, it is essential that the materials and the circuit being constructed be free from contamination. The numerous steps that are required in producing the silicon wafer and fabricating the circuit render contamination inevitable unless significant precautions and procedures are utilized.
In the art of manufacturing the silicon wafers, the silicon crystal is first grown. The crystal is cropped for specification, and then ground into a cylinder. The cylinder is sliced into thin slices (i.e., wafers), with the thickness of the slices being dependent upon the diameter of the cylinder. The slices are then lapped flat, etch ground and contoured, and then initially cleaned. They are then etched for smoothness and to remove damage, thereafter cleaned again, then polished, and then etched and cleaned. In the latter cleaning process, a hydrogen peroxide solution is used to provide a thin oxide coating (e.g., about 25 to 30 angstroms) and the item is rinsed with a purified water solution. Thereafter, the wafers are packaged in a sterile manner and shipped for circuit fabrication.
Circuit fabrication is a complex process which requires numerous steps in which thin film materials are layered on the silicon wafer. During the fabrication process, bare portions of the silicon wafer surface may become exposed. These exposed areas become critical functioning parts of the circuit and if the bare silicon is contaminated in any manner, there may be a subsequent circuit failure which cannot be detected until the circuit is tested. For example, not only is there a critical interface between the bare silicon and a subsequent oxide layer thereon, but there is also a critical interface between the silicon oxide and a polysilicon coating; between the polysilicon coating and the silicide or metallic coating; and many other interfaces. It is essential that the bare silicon and the interfaces be free from contaminants.
Some contamination has more negative effects on the functionality of the circuit elements than other contamination. In general, the impact can be defined as either lithographically blocking or chemically contaminating. Particles which are approximately 10 percent to 30 percent of the minimum feature size and optically opaque may significantly alter the functional performance of the circuit element because they alter the lithographic reproduction process. Even smaller particles reduce yield if they chemically alter the silicon near the front surface, or the composition of the films being deposited on the silicon wafer. The greatest problems involve degradation of the quality of the oxide used for gate elements or the polysilicon deposition used for the gate plates or contacts. These contamination problems may not cause immediate functional failure and loss of die yield, but may show up as oxide or contact reliability problems after the die is shipped to the customer.
Typically, deionized water is used to clean the wafer during the fabrication of the circuit, including cleaning the bare silicon, and cleaning the material interfaces during the fabrication of the device.
The silicon wafer and the entire integrated circuit process is oxide compatible. It is, therefore, an object of the invention to condition the silicon wafer during fabrication of the integrated circuit, in a manner so that it has the cleanest possible oxide layer on the bare silicon and so that it is most receptive to the next layer to be applied.
Another object of the present invention is to provide a process for manufacturing integrated circuits in which the wafer is conditioned during critical interfaces, to minimize problems at the interfaces and produce a better yield.
Other objects of the present invention will become apparent as the description proceeds.